Osteoporosis is a disease characterized by a deficiency of bone that affects both the protein matrix and the mineral fraction, resulting in a decrease in the resistance of bones to fracture. The current method of diagnosis is by means of dual energy x-ray absorptiometry (DEXA), which provides a quantitative measurement of the amount of mineral present in bone and allows determination of fracture risk at a measured site. A decrease in bone mineral density (BMD) as measured by DEXA is the current method of diagnosing osteoporosis and predicting fractures. See, for example, Nevitt and Cummings (1993) J. Am. Geriatr. Soc. 41:1226; and Parfitt (1993) Calcif. Tissue Int. 53:S82. However, the lack of perfect correlation between bone mineral density and bone fractures suggests that low bone mineral density is not the only cause of fragile bones (Ott (1993) Calcif. Tissue Int. 53(Suppl.):S7). Thus, while the degree of mineralization is the current standard by which osteoporosis is diagnosed, it is unable to detect bone fragility due to deficiency in protein matrix.
Bone is a composite material, comprising mineral, organic, and water phases (Katz (1971) J. Biomech. 4:455). The mineral phase, mainly hydroxyapatite (HA), imparts compressive strength, while the organic phase, collagen, imparts flexibility. Wang et al. (1998) Bone 23:67 have shown that with increasing age, the fracture toughness of bone is decreased and its microhardness increased without significant changes in BMD. McCalden et al. reported similar findings, indicating that even without significant changes in BMD, the tensile strength of bone can decrease with age due to increased porosity (McCalden et al. (1993) J. Bone Joint Surg. 75A:1193). There is now a belief that the organic phase of bone plays a significant role in osteoporosis. Kovach et al. have shown that changes in the structural characteristics of the collagen network detected using a laser fluorescence technique correlate significantly with bone fracture toughness (Kovach et al. (1997) Proceedings of the 43th Annual Meeting of the Orthopaedic Research Society, San Francisco, Calif., 22:37). This work is supported by other findings demonstrating that the organic phase of bone is responsible for much of its ability to resist fracture (Wang et al. (1998) Proceedings of the 44th Annual Meeting of the Orthopaedic Research Society, New Orleans, La.; and Wang et al. (2002) Bone 31:1). Mansell and Bailey found that collagen in osteoporotic bone is not normal but instead contains higher levels of lysine hydroxylation and modified cross-linking (Mansell and Bailey (2003) Int. J. Biochem. Cell Biol. 35:522). This and other studies have shown that osteoporosis has a degenerative effect on protein production in bones with increased immature collagen cross-links, increased collagen synthesis and degradation (increased turnover despite overall loss of collagen), as well as reduced mineralization (Oxlund (1996) Bone 19:479; and Bailey (2002) J. Musculoskel. Neuron Interact. 2:529). The increased hydroxylation leads to the formation of finer fibrils with altered crosslinks, and reduced calcification, which further contributes to the fragility of the bone.
One study examined the calcium and magnesium levels in bone and nails (Vecht-Hart et al. (1995) Clin. Chim. Acta 236:1). No correlation was found to exist between the two. Other research examined the relationship between mineral concentrations in nail and bone, and the results have suggested that significant correlations exist between zinc levels and BMD (r=−0.399) and between the ratio of Zn/Ca to BMD (r=0.421) (Karita and Takano (1994) Nippon Koshu Eisei Zasshi 41:759). Nevertheless, these assays all examine the inorganic component of bone and nails, and do not correlate changes in the protein chemistry or structure that may also be present in the disease state.
Bone densitometry is the current gold standard for diagnosis of bone diseases such as osteoporosis. However, this method is limited to measuring bone mass, and it does not take into consideration the microarchitecture of the bone, the crystal organization, size and shape, the connectivity of the trabecullar network, and the structure of the bone proteins. Moreover, DEXA is a relatively expensive diagnostic procedure that exposes the patient to potentially harmful x-rays; thus it cannot be used for mass screenings, such as at routine checkups. Therefore, clinicians risk under diagnosing patients at risk for fracture because the bone disease is often unrecognized until a fracture occurs, or because bone mineral density does not always correlate with a risk of fragile bones even when DEXA is used. The alternative of obtaining collagen from patient's bones is an even more expensive and risky procedure. Thus, clinicians need new, low-risk methods to diagnose patients that are at an increased risk of bone fracture.